1. Field of the invention
The present invention concerns a device for coding-decoding a binary digital signal bit stream for an "OQPSK" digital modulator-demodulator with four phase states.
It applies more particularly to the implementation of modulation and demodulation stages of digital radio transmitter-receivers designed to transmit binary digital bit streams utilizing the modulation method with four phase states known as "Offset Quadrature Phase Shift Keying" ("OQPSK").
2. Description of the Prior Art
It is known to place on the input side of each modulator of a digital radio transmitter operating in "OQPSK" mode a demultiplexer whose function is to divide the binary digital bit stream applied to the input of the modulation stage into two digital bit streams with a bit rate equal to half the bit rate of the digital bit stream of the signals applied to the input and a transition coder with the objective of grouping in pairs the binary symbols or consecutive bits of the signal to be transmitted in order to obtain a representation of the signal in a quaternary code of which each state is utilized in the modulator to phase modulate a fixed frequency signal. As a general rule, the fixed frequency signal is phase-shifted by .pi./2 in the modulator to form two carriers phase-shifted .pi./2 relative to one another. The carriers obtained are respectively applied to the first modulation inputs of two ring modulators of which the second inputs receive, in the case of one of them, the odd ranked bits of the digital bit stream applied to the input, the duration being extended by twice the duration of a symbol of the digital bit stream, and, in the case of the other of them, the even ranked bits of the digital bit stream applied to the input, their duration also being extended by twice the duration of a symbol. The outputs of the ring modulators are applied to the inputs of an adder which, in accordance with the four possible states resulting from the association of two even and odd bits, delivers a signal of frequency F phase-shifted by (2k+1) .pi./4 relative to the fixed frequency signal.
At the receiving end, the quaternary modulation phases of the message are recovered using a phase demodulator which carries out a first multiplication of the recovered carrier with the received phase modulated signals and a second multiplication of the recovered carrier phase-shifted by .pi./2 with the received phase modulated signals. The results of these multiplications are applied to the input of a decoder followed by a multiplexer which reconstitutes the original binary digital bit stream applied to the input of the modulation stage.
The utilization of coder-decoders of the aforementioned type in fixed radio stations does not present any problems. On the other hand, when they equip mobile relay stations, it is necessary to provide relatively complex and expensive doubt resolving devices to reconstitute the symbols of the transmitted messages, since in this case, especially when the relay station first goes into service, there are always uncertainties of at least 90.degree. or .pi./2 radians with regard to the phase of the recovered carrier and several uncertainties of 180.degree. or .pi. radians due to the translation of the frequency of the received carrier into an intermediate frequency which takes place in the receive stage preceding the demodulation stage of the receiver, this translation being effectively accompanied in phase modulation by an inversion of the spectrum of the demodulated carrier according to whether the intermediate frequency is obtained by means of a heterodyne frequency higher than or lower than the frequency of the carrier transmitted by the radio channel.
The objective of the invention is to alleviate the aforementioned disadvantages.